XYZ

Huh, I guess I'm wrong on the infect sliver. My bad. And I can't take credit for the Commander movement, I just know that it's a common thing to do.

The shock lands will speed it up, though. The scry is nice, but speed is more important in a sliver deck.

Mutavault?

I suggest adding, assuming I didn't miss it, Shadow sliver or the sliver that give infect. Also, move Overlord to your sideboard so that its easier to see that he's the commander.

My bad, I misread them. The deck is focused on slivers, technically, and Hive Stirrings summons tokens, for less than what Sliver Queen does. Even though it only works once. And you can easily win the game without the tokens from either card anyways.

I would suggest dropping Caller of Songbirds for cards that get you more tokens. Also, Intangible Virtue is a staple of token decks.

I suggest dropping 2 Thruns, since he's so hard to kill and the Legend Rule makes more than one in your hand useless.

Also: Why use scry lands? You could use shock instead.

I suggest dropping Sliver Queen for Hive Stirrings. Maybe drop one of the Parallel lives, since you don't have much focus on the tokens. Also, I'm curious: Why use scry lands instead of shock or filter?

Remove Darksteel Forge in favor of Manaweft, and the Sliver Constructs for Hive Stirring

You probably won't need four Obzedats.

Ah, OK. I hadn't put together that Varolz was the key. That makes this deck that much better, actually.

Also, you might want to look into damagers from sacs. Bloodthrone Vampire, for instance.

I'm thinking some of the new Golgari cards would be nice additions.

I agree that Voyage's End would be better

I'd say you don't want fetch lands, but fetch spells. There are some better ones that Farseek, and you'll want something to get that combo out.

I always loved vampires.

What's the point of Druid's Deliverance if you can't use half the card? I understand you need more negate damage, but bounce (can't help but agree with Adam) and Essence Scatter would probably be more useful.

Looks pretty good. I have three suggestions: Varolz, you may want to drop two of them, as the legends rule makes it practically useless to have more than two. Thought Seize, nice to help against combos, and the life for Death's Shadow. Sheltering Word, though the life is nice, it messes with the Death's Shadow.

Have you thought about adding Goblin Grenade? It's great to combo with Goblin Arsonist.